She doesn't mess around when it comes to playing cards.
In this sentence, “doesn't mess around” means “she's taking this seriously”?
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It only takes a minute to sign up.Sign up to join this community
Yes, your interpretation is correct in this context. Really, "to mess around" just means to fool around. The opposite of that, which is "doesn't mess around" in more formal terms is that the person who doesn't mess around cares about the action or event and doesn't take it lightly. He or she feels that the action or event is important to them.
Some supporting details:
Merriam-Webster Dictionary defines "to mess around" here: Mess Around Definition
to waste time : dawdle, idle
Collins Dictionary defines "to mess around" here: Mess Around Definition
you spend time doing things without any particular purpose or without achieving anything
As you can see, both give the idea that such a phrase describes the action of wasting idle time behaving in a silly or carefree way. The exact opposite would equate to taking something very seriously and using the time with a particular intent.